Braking force distribution control system

ABSTRACT

An arrangement for controlling the braking force applied to a rear wheel in a certain relationship with a braking force applied to a front wheel so as to provide an ideal braking force distribution includes an auxiliary power source for discharging a power pressure and a dynamic hydraulic braking pressure regulator for regulating the power pressure to a pressure regulated in a certain relationship with the hydraulic braking pressure discharged from a master cylinder. Pressure control valves are disposed in a hydraulic circuit which communicates the pressure regulator with a rear wheel cylinder. The control valves control the hydraulic pressure in the rear wheel cylinder in a certain relationship with the hydraulic pressure in the front wheel cylinder.

This application is a divisional of application Ser. No. 08/144,434,filed Nov. 2, 1993 now U.S. Pat. No. 5,547,264.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a braking force distribution controlsystem for controlling a braking force applied to a rear wheel in abraking operation of a vehicle, and more particularly to the controlsystem for controlling the braking force applied to the rear wheel in acertain relationship with the braking force applied to a front wheel.

2. Description of the Prior Art

In general, when a running vehicle is braked, axle loads applied to thefront and rear portions of the vehicle respectively will be differentfrom each other due to the moving load caused by the braking operation.Therefore, the braking force applied to a front wheel and the brakingforce applied to a rear wheel for locking all the wheels simultaneouslyare not in direct proportion to each other, but in such a relationshipas indicated by a one-dotted chain line in FIG. 13. This relationship isknown as an ideal braking force distribution which varies depending uponthe condition with or without load. The distribution under the conditionwith load will be the one as indicated by a two-dotted chain line inFIG. 13.

If the braking force applied to the rear wheel exceeds the braking forceapplied to the front wheel, the directional stability of the vehiclewill be deteriorated. In order to keep the braking force applied to therear wheel lower than that applied to the front wheel and provide adistribution in close proximity to the ideal braking force distribution,a proportioning valve is provided between the rear wheel brake cylinderand the master cylinder. With this arrangement, a distributioncharacteristic has a break point as indicated by a phantom line in FIG.13. When the difference of the loads applied to the inner and outerwheels of a turning vehicle is taken into consideration for example, itis necessary to reduce the braking force applied to the rear wheel muchlower than the braking force applied to the front wheel. In addition,when the loadage is large, the distribution will be far remote from theideal braking force distribution. Therefore, a load sensingproportioning valve is installed in a truck or the like to provide adistribution characteristic with a break point which varies in responseto the loadage.

In Japanese Publication for Opposition No. 51-40816, it is proposed tovary the break point of the distribution characteristic for theproportioning valve by a pneumatic actuator, in accordance with a resultof a comparison between the rotational speeds of the front and rearwheels. In that publication, is disclosed a structure which provides ahigh break point when the rotational speed of the rear wheel is higherthan that of the front wheel, and provides a low break point when therotational speed of the rear wheel is lower than that of the frontwheel.

According to the above-described arrangement in the prior art, however,it is insufficient to provide a proximate ideal braking forcedistribution with respect to the front and rear wheels of the vehicle.Since the braking force distributed to the rear wheel is reduced in theprior art, a large force will have to be applied to the brake pedal forproducing a desired deceleration of the vehicle. Otherwise, a large loadwill be applied to the front wheel braking system. Or, a large brakingforce will be distributed to the rear wheel, so that the rear wheel willbe likely to be locked.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide abraking force distribution control system for use in a vehicle, andprovide a proximate ideal braking force distribution by a simplearrangement.

It is another object of the present invention to provide a stablebraking force distribution control irrespective of variable wheel speedsof front and rear wheels of the vehicle.

It is a further object of the present invention to provide a smoothbraking operation for a transitional period from the braking forcedistribution control to the normal braking operation at the terminationof the braking force distribution control.

In accomplishing the above and other objects, a braking forcedistribution control system is provided for controlling a braking forceapplied to a rear wheel of an automotive vehicle in a certainrelationship with a braking force applied to a front wheel of theautomotive vehicle. The system includes a front wheel brake cylinderwhich is operatively connected to the front wheel for applying thebraking force thereto, and a rear wheel brake cylinder which isoperatively connected to the rear wheel for applying the braking forcethereto. A master cylinder is provided for pressurizing a brake fluidfed from a reservoir and supplying a hydraulic braking pressure to thefront wheel brake cylinder in response to depression of a brake pedal.Pressure control valves are disposed in a hydraulic circuit whichcommunicates the pressure regulator with the rear wheel brake cylinderto control the hydraulic braking pressure in the rear wheel brakecylinder. An auxiliary power source is communicated with the reservoirto pressurize the brake fluid fed from the reservoir and discharge apower pressure. A dynamic hydraulic braking pressure regulator such as ahydraulic booster is communicated with the auxiliary power source toregulate the power pressure to a pressure regulated in a certainrelationship with the hydraulic braking pressure discharged from themaster cylinder in response to depression of the brake pedal. And, acontroller is provided for actuating the pressure control valves tocontrol the hydraulic braking pressure in the rear wheel brake cylinderin a certain relationship with the hydraulic braking pressure in thefront wheel brake cylinder.

The braking force distribution control system may further include wheelspeed sensors for detecting wheel speeds of the front wheel and the rearwheel, and a comparator for comparing the wheel speeds of the frontwheel and the rear wheel detected by the sensors. And, the controller isarranged to actuate the pressure control valves in response to theresult of comparison made in the comparator so as to control thehydraulic braking pressure in the rear wheel brake cylinder in a certainrelationship with the hydraulic braking pressure in the front wheelbrake cylinder. A certain condition for terminating the control of thehydraulic braking pressure by the controller is determined. Then, apulse increase mode is provided to repeat holding and increasingoperations of the hydraulic braking pressure in the rear wheel brakecylinder, and further a special pulse increase mode is provided toincrease the hydraulic braking pressure in the rear wheel brake cylinderat an increasing rate raised in accordance with a lapse of time afterthe terminating condition has been fulfilled.

The braking force distribution control system may further include wheelspeed sensors for detecting wheel speeds of the front wheel and the rearwheel, and may be arranged to provide a certain increasing rate for eachof the wheel speeds of the front and rear wheels detected by the sensorsand calculate a first set speed on the basis of the increasing rate foreach of the front and rear wheels, provide a certain decreasing rate foreach of the wheel speeds of the front and rear wheels detected by thesensors and calculate a second set speed on the basis of the decreasingrate for each of the front and rear wheels, and select a medium value ofthe first set speed, the second set speed and the wheel speed of thefront wheel or rear wheel detected by the sensors to set a standardspeed for each of the front and rear wheels. It is so arranged that thestandard speed of the front wheel and the standard speed of the rearwheel are compared, and that the controller actuates the pressurecontrol valves in response to the result of comparison to control thehydraulic braking pressure in the rear wheel brake cylinder in a certainrelationship with the hydraulic braking pressure in the front wheelbrake cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

The above stated objects and following description will become readilyapparent with reference to the accompanying drawings, wherein likereference numerals denote like elements, and in which:

FIG. 1 is a general block diagram illustrating a braking forcedistribution control system according to an embodiment of the presentinvention;

FIG. 2 is a block diagram illustrating the arrangement of the electroniccontroller shown in FIG. 1;

FIG. 3 is a flowchart showing the operation of the braking force controlaccording to an embodiment of the present invention;

FIG. 4 is a flowchart showing the operation of the braking forcedistribution control according to an embodiment of the presentinvention;

FIG. 5 is a flowchart showing the operation of the braking forcedistribution control of the rear right wheel according to an embodimentof the present invention;

FIG. 6 is a flowchart showing the calculation of constant for thebraking force distribution control according to an embodiment of thepresent invention;

FIG. 7 is a flowchart showing the calculation of standard speeds for thebraking force distribution control according to an embodiment of thepresent invention;

FIG. 8 is a flowchart showing the determination of starting conditionfor the braking force distribution control according to an embodiment ofthe present invention;

FIG. 9 is a flowchart showing the operation of the special pulseincrease control for the braking force distribution control according toan embodiment of the present invention;

FIG. 10 is a diagram showing a control map for the braking forcedistribution of the rear right wheel according to an embodiment of thepresent invention;

FIG. 11 is a diagram showing the variation of wheel speed of the rearright wheel according to an embodiment of the present invention;

FIG. 12 is a diagram showing the variations of hydraulic pressure inrear and front wheel brake cylinders according to an embodiment of thepresent invention;

FIG. 13 is a diagram showing the braking force distribution operation inan embodiment of the present invention comparing with a prior art;

FIG. 14 is a general block diagram illustrating a braking forcedistribution control system according to another embodiment of thepresent invention; and

FIG. 15 is a general block diagram illustrating a braking forcedistribution control system according to a further embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is schematically illustrated a braking forcedistribution control system according to the present invention, whichcontrols a braking force applied to each of wheels FR, FL, RR, RL of avehicle individually depending upon a braking condition.

Referring to FIG. 1, a hydraulic pressure generator 2 comprises a tandemmaster cylinder 2 and a hydraulic booster 5 operated in response todepression of a brake pedal 3. An auxiliary pressure source 20 isconnected to the hydraulic booster 5, and these are connected to a lowpressure reservoir 4 respectively. Wheel brake cylinders 51 to 54 areconnected to the wheels FR, FL, RR, and RL, respectively. The wheel FRdesignates the wheel at the fore right side as viewed from the positionof a driver's seat, the wheel FL designates a wheel at the fore leftside, the wheel RR designates a wheel at the rear right side, and thewheel RL designates a wheel at the rear left side. In the presentembodiment, a so-called Y circuit system has been employed for the rearwheels RR, RL as shown in FIG. 1, while a so-called X circuit system maybe employed. A rear wheel drive system, where the rear wheels RR, RL aredriven, has been employed in the present embodiment. However, a frontwheel drive system may be employed.

There are disposed solenoid valves 61, 62 in hydraulic circuits forconnecting a pressure chamber 2a of the master cylinder 2 to the frontwheel brake cylinders 51, 52, respectively. The solenoid valves 61, 62are connected to the hydraulic booster 5 through a normally opensolenoid valves 31, 33, and also connected to a normally closed solenoidvalves 32, 34, which are connected to the low pressure reservoir 4. Aproportioning valve 6 and a solenoid valve 63 are connected to apressure chamber 2b of the master cylinder 2. In the hydraulic circuitsfor connecting the solenoid valve 63 and the rear wheel brake cylinders53, 54, there are disposed normally open solenoid valves 35, 37respectively. The wheel brake cylinders 53, 54 are connected to the lowpressure reservoir 4 through normally closed solenoid valves 36, 38respectively. These solenoid valves 35 to 38 serve as pressure controlvalve means according to the present invention.

The auxiliary pressure source 20 includes a pump 21, an accumulator 22and a relief valve 23. The pump 21 is driven by an electric motor 24, sothat a brake fluid is fed from the low pressure reservoir 4 and raisedto a predetermined pressure and supplied through a check valve 25 to theaccumulator 22 to be stored therein. A relief valve 23 is provided forreturning the brake fluid to the low pressure reservoir 4 when thedischarged hydraulic braking pressure exceeds a predetermined pressure,to thereby decrease the hydraulic braking pressure. At the output sideof the accumulator 22 are disposed a pressure sensor 46 which providesan output signal linear to the hydraulic pressure, and a low pressureswitch 47 which turns on when it detects a pressure lower than apredetermined value. Thus, a so-called power pressure is discharged fromthe auxiliary pressure source 20, and supplied to the hydraulic booster5.

The hydraulic booster 5, which serves as the dynamic hydraulic brakingpressure regulating means according to the present invention, isarranged to regulate the hydraulic braking pressure discharged from theauxiliary power source 20 by a spool valve (not shown) which is actuatedin response to depression of the brake pedal 3, and boost the mastercylinder 2 by the regulated pressure. The hydraulic boosters aredisclosed in Japanese Patent Laid-open Publication Nos. 64-47664,64-47665 and 64-74156, for example, each disclosure of which isincorporated by reference in its entirety. The hydraulic booster 5 ofthe present embodiment is so arranged that the discharged pressure isregulated to provide a pressure higher by a certain rate (e.g., 20%)than the pressure discharged from the master cylinder 2 (therefore,regulated to a pressure corresponding to 120% of the pressure dischargedfrom the master cylinder 2). Then, the hydraulic circuits are soarranged that the regulated pressure is fed to the wheel brake cylinders51 to 54, through the solenoid valves 31, 33 and the solenoid valves 63,35, 37. There is provided a regulated pressure switch 48 which turns onwhen the regulated pressure exceeds a predetermined pressure. Aregulator as disclosed in the above-identified publications may besubstituted for the dynamic hydraulic braking pressure regulating meansaccording to the present invention, so as to be constructed separatelyfrom the master cylinder 2.

Accordingly, the solenoid valve 63, which serves as a changeover valveaccording to the present invention, is disposed between the mastercylinder 2 and the solenoid valves 35 to 38 which serve as pressurecontrol valve means according to the present invention. And, theproportioning valve 6 is disposed between the solenoid valve 63 and themaster cylinder 2. The hydraulic circuits at the drain side of thesolenoid valves 32, 34 and the solenoid valves 36, 38 are connected tothe low pressure reservoir 4, which receives the brake fluid returnedfrom the solenoid valves 32, 34, 36, 38 through the hydraulic circuitsat their drain sides, and stores the brake fluid for supplying the sameto the master cylinder 2 and etc.

Each of the solenoid valves 61 to 63 is a three ports-two positionssolenoid operated changeover valve, and is in its first operatingposition as shown in FIG. 1 when a current is not fed to its solenoidcoil, so as to allow the wheel brake cylinders 51 to 54 to communicatewith the master cylinder 2 and prevent them from communicating with thehydraulic booster 5. When the current is fed to the solenoid coil, eachsolenoid valve is shifted to a second operating position, i.e., changedover to the right side in FIG. 1.

Each of the solenoid valves 31 to 38 is a two ports-two positionssolenoid operated changeover valve, and is in its first operatingposition as shown in FIG. 1 when a current is not fed to its solenoidcoil, so that each of the wheel brake cylinders 51 and 52 iscommunicated with the hydraulic booster 5 provided that the solenoidvalves 61, 62 are in the second position, and the wheel brake cylinders53, 54 are connected with the hydraulic booster 5. When the current isfed to the solenoid coil, each solenoid valve is changed over to itssecond operating position, so that each of the wheel brake cylinders 51and 52 is shut off from the communication with the hydraulic booster 5through the solenoid valves 61, 62, and is communicated with the lowpressure reservoir 4 through the solenoid valves 61, 62, and each of thewheel brake cylinders 53, 54 is shut off from the communication with thehydraulic booster 5 through the solenoid valve 63, and is communicatedwith the low pressure reservoir 4. Other check valves as shown in FIG. 1permit the brake fluid to return from each of the wheel brake cylinders51 to 54 to the hydraulic booster 5, and blocks the counterflow of thebrake fluid.

Accordingly, with each of the solenoid valves 61 to 63 energized orde-energized, the communication between the wheel brake cylinders 51 to54 and the master cylinder 2 or the hydraulic booster 5 is changed over.And, with each of the solenoid valves 31 to 38 energized orde-energized, the hydraulic braking pressure in each of the wheel brakecylinders 51 to 54 is decreased, held or increased. Namely, when thecurrent is not fed to the solenoid coil of each of the solenoid valves31 to 38, the hydraulic braking pressure is supplied from the hydraulicbooster 5 to each of the wheel brake cylinders 51 to 54 to increase thehydraulic braking pressure in each wheel brake cylinder. On the otherhand, when the current is fed to the solenoid coil, each of the wheelbrake cylinders 51 to 54 is communicated with the low pressure reservoir4 to decrease the hydraulic braking pressure in each wheel brakecylinder. Further, when the current is fed only to the solenoid coils ofthe solenoid valves 31, 33, 35 and 37, the hydraulic braking pressure ineach wheel brake cylinder is held. Accordingly, by adjusting the periodfor energizing or de-energizing the solenoid valves 31 to 38, aso-called pulse increase mode (i.e., step increase mode), or a pulsedecrease mode is provided so as to gradually increase or decrease thehydraulic braking pressure.

The above-described solenoid valves 31 to 38 and solenoid valves 61 to63 are electrically connected to the electronic controller 10 whichcontrols the operation of those solenoid valves. The electric motor 24is also electrically connected to the electronic controller 10, so thatthe operation of the electric motor 24 is controlled by the electroniccontroller 10. At the wheels FR, FL, RR and RL, there are provided wheelspeed sensors 41 to 44 respectively, which are electrically connected tothe electronic controller 10, and by each of which a signalcorresponding to a rotational speed of each road wheel, i.e., a wheelspeed signal is fed to the electronic controller 10. There is alsoprovided a brake switch 45 which turns on when the brake pedal 3 isdepressed, and turns off when the brake pedal 3 is released, and whichis electrically connected to the electronic controller 10. Further, thepressure sensor 46, low pressure switch 47 and regulated pressure switch48 are electrically connected to the electronic controller 10.

As shown in FIG. 2, the electronic controller 10 is provided with aone-chip microcomputer 11, which includes a central processing unit orCPU 14, a read-only memory or ROM 15, a random access memory or RAM 16and a timer 17, which are connected with an input port 12 and an outputport 13 via a common bus to execute the input/output operations relativeto external circuits. The signals detected by each of the wheel speedsensors 41 to 44, the brake switch 45, the pressure sensor 46, the lowpressure switch 47 and the regulated pressure switch 48 are fed to theinput port 12 via respective amplification circuits 18a to 18h and thento the CPU 14. Then, a control signal is fed from the output port 13 tothe electric motor 24 via a drive circuit 19a, and control signals arefed to the solenoid valves 31 to 38 and solenoid valves 61 to 63 via therespective drive circuits 19b to 19l. In the microcomputer 11, the ROM15 memorizes a program corresponding to flowcharts shown in FIGS. 3 to9, the CPU 14 executes the program while the ignition switch (not shown)is closed, and the RAM 16 temporarily memorizes variable data necessaryfor executing the program.

The program routine executed by the electronic controller 10 for thebraking force distribution control will now be described with referenceto FIGS. 3 to 9. FIG. 3 is a flowchart showing a main routine executedin accordance with a program of one embodiment of the present invention.The program routine corresponding to the flowchart as shown in FIG. 3starts when the ignition switch (not shown) turns on, and provides forinitialization of the system at Step 100 to clear various data. Then,the program proceeds to Step 101 where the wheel speeds Vwff, Vwfr,Vwrr, Vwr1 are calculated on the basis of output signals from the wheelspeed sensors 41 to 44, and these wheel speeds are differentiated toobtain wheel accelerations DVwfr, DVwf1, DVwrr, DVwr1. Alternately, anacceleration sensor may be provided for producing the wheel accelerationsignals. And, at Step 103 an estimated vehicle speed Vso is calculatedon the basis of the wheel speeds, and as its differential value, avehicle acceleration DVso is calculated. The estimated vehicle speed Vsocorresponds to the value representing a vehicle speed and is calculatedas follows. That is, a vehicle speed in braking operation is set to avalue calculated on the assumption that the vehicle speed is reducedwith a predetermined deceleration from the vehicle speed correspondingto the wheel speed in braking operation, and then, if the wheel speed ofany one of four wheels exceeds the wheel speed corresponding to thevehicle speed as set above, the vehicle speed is reset to a valuecalculated on the assumption that the vehicle speed of the valuepreviously set is reduced with the predetermined deceleration again fromthe vehicle speed corresponding to the exceeded wheel speed. Theestimated vehicle speed Vso is, therefore, the same as that provided forthe conventional anti-skid control.

Next, at Step 200, the solenoid valve 63 is energized to be placed inthe second operating position on a certain condition where the brakeswitch 45 turns on with the brake pedal 3 depressed, for example, sothat the wheel brake cylinders 53, 54 are blocked from the communicationwith the master cylinder 2, and communicated with the hydraulic booster5. Then, the program proceeds to Step 500, where it is determinedwhether the condition for initiating the anti-skid control operation isfulfilled or not. If it is determined that the condition is fulfilled tobe in the anti-skid control mode, the solenoid valves 61, 62 are shiftedto the second operating positions and the solenoid valves 31 to 38 areactuated to perform the anti-skid control operation at Step 600. If itis determined not to be in the anti-skid control mode at Step 500, thenthe program proceeds to Step 700 where it is determined whether thebraking force distribution control mode is to be selected. If it isaffirmative at Step 700, the program further proceeds to Step 800,otherwise it proceeds to Step 300. Whether the braking forcedistribution control mode is to be selected or not is determined on thebasis of various conditions of the vehicle in the braking condition. Forexample, it is determined to initiate the braking force distributioncontrol, provided that all the conditions are fulfilled, such that theanti-skid control system is normal, that the braking force distributioncontrol system is normal, that the rear wheels RR, RL are not under theanti-skid control, and that the solenoid valve 63 is energized. Then,the program proceeds to Step 800 where the braking force distributioncontrol is executed, and thereafter the program returns to Step 101.

At Step 300, it is determined whether a certain braking operation hasbeen made or not. That is, after the brake pedal 3 was depressed, thewheel speed Vwfr (Vwf1) of the front wheel FR (FL) is decreased to belower than the estimated vehicle speed Vso, and the wheel accelerationDVw, which is a differential value of the wheel speed, becomes lowerthan a predetermined acceleration (including deceleration) G1, then itis determined that a pre-control output is to be permitted, so that theprogram proceeds to Step 400 where a pre-control pulse increase controlis initiated, otherwise it returns to Step 101. The pre-control pulseincrease control has been employed in the prior anti-skid controlsystem, in such a manner that the solenoid valves 31, 33, 35, 37 areenergized or de-energized to repeat the holding operation and increasingoperation of the hydraulic braking pressure. Therefore, it is alsocalled as a pre-control hold control. After the pre-control pulseincrease control is terminated, the program returns to Step 101.

In the above-described operation, a fail-safe function is provided. Thatis, when some abnormality is found in the braking force distributioncontrol system, the solenoid valve 63 will be de-energized to return toits first operating position as shown in FIG. 1, and the solenoid valves35, 37 will be placed in their open positions, so that the wheel brakecylinders 53, 54 will be communicated with the master cylinder 2 throughthe proportioning valve 6. Consequently, the rear wheels RR, RL areapplied with the braking force determined on the basis of the brakingforce distribution provided as in the prior art.

The braking force distribution control in Step 800 is executed accordingto a flowchart as shown in FIG. 4, wherein various constants fordetermining the starting condition of the braking force distribution areset at Step 801 which will be described later in detail with referenceto FIG. 8. Then, at Step 802, standard speeds Vwsfr, Vwsf1, Vwsrr, Vwsr1are calculated on the basis of the wheel speeds Vwfr, Vwf1, Vwrr, Vwr1of the wheels FR, FL, RR, RL by a predetermined operation which will bedescribed later with reference to FIG. 7. The differences between thestandard speeds of the front and rear wheels (Vwsrr-Vwsfr),(Vwsr1-Vwsf1) are calculated at Step 803 to provide the standard speeddifferences DVwsrr, DVwsr1, respectively. Then, the program proceeds toSteps 804, 805 where the braking force distribution control for the rearwheels RR, RL are executed.

FIG. 5 shows a subroutine of Step 804 in FIG. 4 with respect to thebraking force distribution control for the rear wheel RR, which will bedescribed hereinafter, and the braking force distribution control forthe rear wheel RL will be executed as well. At the outset, it isdetermined at Step 820 whether the braking force distribution controlhas been already initiated. If a control flag representing that thedistribution control has been initiated is not set (i.e., "0"), then theprogram proceeds to Steps 821 to 826, and if it is set (i.e., "1"), theprogram proceeds to Steps 827 to 830.

At Step 821, the initiation of the braking force distribution controlwith respect to the wheel RR is determined. As for the startingcondition, there are provided various conditions, such that the standardspeed Vwsrr of the rear wheel RR, for example, is in a certainrelationship with the standard speed Vwsfr of the front wheel FR, thatthe standard acceleration DVso is less than a predetermined value, e.g.,-0.25 G (G: gravitational acceleration), that the brake switch 45 is inits ON condition, and that the estimated vehicle speed Vso is greaterthan a predetermined speed, e.g., 15 km/h. The determination of thestarting condition will be described later in detail with reference toFIG. 8. With all the conditions fulfilled, it is determined that thebraking force distribution control may be initiated, so that the controlflag is set to "1" at Step 822, and the program further proceeds to Step823. Otherwise, the program returns to the routine in FIG. 4.

At Step 823, a slip rate Sprr or the like is calculated on the basis ofthe aforementioned standard speed Vwsrr or the like, and controlstandard values Tsrr, Dfrr are calculated. The control standard valueDfrr is a variation of the standard speed difference DVwsrr, or adifference between the value in the previous cycle and the value in thepresent cycle, DVwsrr(n)-DVwsrr(n-1). The slip rate Sprr is a slip rateof the standard speed Vwsrr of the rear right wheel RR with respect tothe standard speed Vwsfr of the front right wheel FR,(Vwsrr-Vwsfr)/Vwsfr, which is integrated to provide a value ISprr. Then,the control standard value Tsrr is calculated from a function (Sprr,Dfrr, ISprr).

On the basis of the control standard values Tsrr, Dfrr, a control map isformed as shown in FIG. 10, and the control mode is determined at Step824 in accordance with the control map. In FIG. 10, the ordinaterepresents the control standard value Tsrr which is obtained by addingthe slip rate Sprr and the integrated value ISprr according to thisembodiment, while the abscissa represents the control standard valueDfrr. There are provided two zones of P-zone and D-zone, which aredivided by a line segment for connecting an intersection of X1 (G) andY1 (%) with an intersection of X2 (G) and Y2 (%), and a line segmentparallel with the X-axis (i.e., abscissa). The P-zone represents a zonewhere the pulse increase control mode is selected, while the D-zonerepresents a zone where the pulse decrease control mode is selected. Ineach zone, a period Tb and on-time are set for each control pulsesignal. The period Tb is calculated in accordance with the followingformula:

    Tb=Kb-Kc×L

where L corresponds to a length of a perpendicular from a random pointto the line segment connecting the intersection (X1, Y1) with theintersection (X2, Y2) as shown in FIG. 10, and Kb, Kc are constants. "x"represents multiplication. Accordingly, in response to that controlpulse signal, the pulse decrease control or pulse increase control areexecuted at Step 825 or Step 826 in FIG. 5, respectively.

Referring back to Step 820, if it is determined that the control flag isset to "1", then it is determined at Step 827 whether the terminatingcondition has been fulfilled or not. For the terminating condition,there are provided various conditions, such that the brake switch 45turns off, and that the standard acceleration DVso exceeds thepredetermined value (-0.25 G). If either one of those conditions isfulfilled, it is determined that the braking force distribution controlmay be terminated. Consequently, the control flag is reset to "0" atStep 828 and the program proceeds to Step 829. If the terminatingcondition is not fulfilled, the program proceeds to Step 823 and thebraking force distribution control is continued.

At Step 829 is determined a lapse time after the control flag was reset(hereinafter, simply referred to as a lapse time). If it is determinedthat the lapse time is equal to or greater than the predetermined timeT0, the program proceeds to Step 830, where a normal increase control isexecuted. If it is determined that the lapse time is less than thepredetermined time T0, the program proceeds to Step 860, where a specialpulse increase control is executed, and then proceeds to Step 830. Thespecial pulse increase control will be explained later in detail withreference to FIG. 9.

Referring to FIG. 6, the calculation of constant executed at Step 801 inFIG. 4 for varying the starting condition of the braking forcedistribution control in response to a running road condition will beexplained hereinafter. At the outset, if it is determined at Step 811that the vehicle is running on a rough road, a first set value Vwz1 isset for a bias speed Vwz at Step 812. Otherwise, a second set value Vwz2(less than Vwz1) is set for the bias speed Vwz. The determination of therough road executed at Step 811 is the same as the determination of theroad condition executed for the anti-skid control in the prior art. Forexample, the road condition of the running road is determined inresponse to the number of times which the wheel acceleration exceeded apredetermined value within a certain period of time, as described inJapanese Laid-open Publication No. 3-284463, the disclosure of which isincorporated by reference in its entirety. Then, at Step 812, the biasspeed Vwz is added to the slip rate bias speed (Vwsfr×Spz) to provide aconstant K3 (=Vwz+Vwsfr×Spz). "Spz" represents a slip rate of thestandard speed Vwsrr with respect to the standard speed Vwsfr.

Referring next to FIG. 7, the calculation of the standard speedsexecuted at Step 802 in FIG. 4 will be described with respect to therear right wheel RR. The standard speeds for the remaining wheels of thevehicle will be calculated in the same manner as described below. Thewheel speed Vwrr of the wheel RR calculated at Step 101 is storedsequentially in the memory at a predetermined operation period, thevalue Vwrr(n) in the present cycle (n) is set for a value A at Step 841.Next, a predetermined value (Rup×t) is added to the value Vwrr(n-1) inthe previous cycle (n-1) to provide a value B at Step 842. Then, apredetermined value (Rdn×t) is subtracted from the previous valueVwrr(n-1) to provide a value C at Step 843. Thereafter, a medium valueof the values A, B and C is selected at Step 844 to provide the standardvalue Vwsrr. "Rup" is a value for setting a limitation of anacceleration of the wheel speed Vwrr, or an increasing rate of the wheelspeed Vwrr, and set to 2 G (G: gravitational acceleration), for example."t" is an operation period of a calculation cycle which is set to 10msec, for example. "Rdn" is a value for setting a limitation of adeceleration of the wheel speed Vwrr, or a decreasing rate of the wheelspeed Vwrr. In the present embodiment, "Rdn" is set to a total value ofthe acceleration DVwrr and a value (R1) of its certain rate (i.e.,Rdn=DVwrr+R1), wherein the value R1 is set to a value corresponding to25% of the acceleration DVwrr, for example. In the vehicle which isprovided with the acceleration sensor (not shown), however, "Rdn" is setto a total value of a value GO detected by the acceleration sensor and acorrection value R0 (i.e., Rdn=G0+R0).

FIG. 8 shows an example for the determination of the starting conditionfor the braking force distribution control executed at Step 821 in FIG.5. It is determined at Step 851 whether the brake switch 45 is in its ONcondition. If it is in its ON condition, the program proceeds to Step852, otherwise it proceeds to the next routine as the starting conditionis not fulfilled. At Step 852, the estimated vehicle speed Vso iscompared with a predetermined speed K1 (e.g., 15 km/h). If the former(Vso) is equal to or greater than the latter (K1), the program proceedsto Step 853, otherwise it is determined that the starting condition isnot fulfilled. Next, at Step 853, the acceleration DVso is compared witha predetermined acceleration K2 (e.g., -0.25 G). If the former (DVso) isequal to or smaller than the latter (K2), the program proceeds to Step854, while if the former exceeds the latter, it is determined that thestarting condition is not fulfilled. Further, at Step 854, the standardspeed Vwsrr of the wheel RR is compared with a predetermined standardvalue (Vwsfr-K3). "K3" corresponds to the constant K3 which iscalculated at Step 814 in FIG. 6. If the former (Vwsrr) is less than thelatter, it is determined that the starting condition is fulfilled, sothat the control flag is set to "1" at Step 822. Otherwise, it isdetermined that the starting condition is not fulfilled.

Referring to FIG. 9, the special pulse increase control executed at Step860 in FIG. 5 will be described hereinafter. At the outset, the on-timeTp of the control pulse signal, which is provided for increasing thehydraulic braking pressure in the wheel brake cylinder 53, is set to apredetermined time (e.g., 6 msec). Then, the program proceeds to Step862, where the lapse time is compared with a predetermined time T1 (lessthan T0). If it is determined that the lapse time is equal to or greaterthan the time T1, the period Tb is set to Tb1 (e.g., 16 msec) at Step863, whereas if it is less than the time T1, the program proceeds toStep 864. Similarly, at Step 864 the lapse time is compared with apredetermined time T2 (less than T1). If it is determined that the lapsetime is equal to or greater than the time T2, the period Tb is set toTb2 (e.g., 32 msec) at Step 865, whereas if it is less than the time T2,the program proceeds to Step 866. At Step 866, the lapse time is furthercompared with the predetermined time T3 (less than T2). If it isdetermined that the lapse time is equal to or greater than the time T3,the period Tb is set to Tb3 (e.g., 64 msec) at Step 867. If it is lessthan the time T3, the period Tb is set to Tb4 (e.g., 128 msec) at Step868. Then, the program proceeds to Step 869, where the pulse increasecontrol is executed on the basis of the control pulse signal with theon-time Tp and the period Tb, so that the period Tb is graduallydecreased with the lapse time increased, to thereby increase theincreasing rate of the hydraulic braking pressure. Accordingly, if it isdetermined at Step 870 that the hydraulic braking pressure Pwrr in thewheel brake cylinder 53 of the rear right wheel RR is equal to thehydraulic braking pressure Pwfr in the wheel brake cylinder 51 of thefront right wheel FR, then the program proceeds to Step 830 in FIG. 5.If the pressure Pwrr has not reached the pressure Pwfr, the programreturns to Step 862 and the above-described operation will be repeated.While the on-time Tp has been set to a constant value in the presentembodiment, it may be of variable values such as the period Tb.

FIG. 11 illustrates the variation of the wheel speed of the rear rightwheel RR in comparison with that of the front right wheel FR accordingto the present embodiment. In FIG. 11, the brake pedal 3 is depressed ata position "a", so that the standard speed Vwsrr begins to be decreased.When the standard speed Vwsrr becomes lower than the standard value(Vwsfr-K3) as indicated by a two-dotted chain line at a position "b",the braking force distribution control for the wheel RR is initiated tostart the operation for limiting the hydraulic braking pressure in thewheel brake cylinder 53. When the standard speed Vwsrr exceeds the upperstandard value as indicated by the one-dotted chain line, for example,the pulse increase operation starts with respect to the wheel brakecylinder 53. Thus, a zone defined between the upper and lower one-dottedchain lines serves as an insensitive zone, whereby a stable controloperation is ensured without being affected by disturbance or noise. InFIG. 11, the distribution control is terminated at a point "c", and thebrake pedal 3 is released at a point "d". Consequently, the brakingforce applied to the vehicle with or without the load is controlled totrace the ideal braking force distribution characteristic, as indicatedby solid lines in FIG. 13.

FIG. 12 illustrates the relationship between the hydraulic brakingpressures Pwfr, Pwrr of the wheel brake cylinders 51, 53 when thebraking force distribution control is executed with respect to the wheelRR. The positions "b" and "c" correspond to the positions "b" and "c" inFIG. 11, respectively. During the period as indicated by an arrow inFIG. 12 after the braking force distribution control was terminated atthe position "c", the special pulse increase control is executed inaccordance with the flowchart as shown in FIG. 9, whereby the increasingrate of the hydraulic braking pressure will be increased in accordancewith the lapse time after the termination of the distribution control.As described before, this special control is arranged to be terminatedwhen the pressure Pwrr in the wheel brake cylinder 53 becomes equal tothe pressure Pwfr in the wheel brake cylinder 51, and shifted to thenormal increase control. That is, even if the pressure Pwrr was set onthe basis of the on-time Tp and the period Tb to exceed the pressurePwfr in the wheel brake cylinder 51, it is so controlled that when thepressure Pwrr becomes equal to that of the pressure Pwfr, the specialpulse increase control will be terminated. Thus, the hydraulic brakingpressure in the rear wheel brake cylinder 53 is not increased rapidlyafter the termination of the braking force distribution control, butgradually increased up to the hydraulic braking pressure in the frontwheel brake cylinder 51, to thereby provide a smooth braking operationfor a transitional period to the normal braking operation.

FIG. 14 illustrates another embodiment of the present invention, whichfurther includes a solenoid valve 64 in the embodiment as shown in FIG.1, to provide a so-called traction control function. That is, in thehydraulic circuit connecting the hydraulic booster 5 with the solenoidvalve 63 in FIG. 1, a solenoid valve 64 of three ports-two positionselectromagnetic changeover valve is disposed, and its one port isconnected to the output side of the accumulator 22. The solenoid valve64 is placed in a first operating position as shown in FIG. 14 when itssolenoid coil is de-energized, to connect the solenoid valve 63 with thehydraulic booster 5 and block the communication between the valve 63 andthe accumulator 22. When the solenoid valve 64 is energized, it isshifted to its second operating position where the communication betweenthe solenoid valve 63 and the hydraulic booster 5 is blocked, and thesolenoid valve 63 is communicated with the accumulator 22.

Accordingly, during the normal braking operation, the solenoid valve 63is placed in the second operating position, and communicated with thehydraulic booster 5 through the solenoid valve 64 which is placed in thefirst operating position, so that the wheel brake cylinders 53, 54 canbe communicated with the hydraulic booster 5 through the solenoid valves35, 37, 63 and 64. When an acceleration slip is detected with respect tothe wheels RR, RL of driven wheels, the solenoid valve 64 is shifted toits second operating position, whereby the wheel brake cylinders 53, 54can be directly communicated with the accumulator 22. Then, the solenoidvalves 35 to 38 are actuated in accordance with the slip conditions ofthe wheels RR, RL, so that the braking force is applied to the wheelsRR, RL to thereby prevent them from rotating extraordinarily. When thebrake pedal 3 is depressed to start braking operation and the brakingforce distribution control is executed, the solenoid valve 64 is placedto its first operating position.

The operation of the solenoid valves 63, 64 will now be explainedsequentially. First of all, in the stopped condition or abnormalcondition, both of the solenoid valves 63, 64 are de-energized, and theaforementioned braking force distribution control is not executed, butthe hydraulic braking pressure in the wheel brake cylinders 53, 54 areregulated by the proportioning valve 6. During the anti-skid controloperation, the solenoid valve 63 is energized, while the solenoid valve64 is de-energized, so that the hydraulic pressure is fed from thehydraulic booster 5 to the wheel brake cylinders 53, 54. During thetraction control operation, both of the solenoid valves 63, 64 areenergized, so that the power pressure is fed from the accumulator 22 tothe wheel brake cylinders 53, 54. And, during the normal brakingoperation, the solenoid valve 64 is de-energized, while the solenoidvalve 63 is energized, so that the regulated pressure is fed from thehydraulic booster 5 to the wheel brake cylinders 53, 54 and theaforementioned braking force distribution control is executed. Thosecontrols are arranged to be executed according to priority, such as theresponse to an abnormal operation to be executed at first, next theanti-skid control, then the braking force distribution control, and thenthe pre-control pulse increase control, and the traction control atlast.

FIG. 15 illustrates a further embodiment of the present invention,wherein a front and rear dividing hydraulic circuit system is employed,so that the rear wheels RR, RL can be controlled independently of eachother. That is, a solenoid valve 65 of a three ports-two positionselectromagnetic changeover valve and a proportioning valve 66 areprovided in addition to the arrangement as shown in FIG. 1. According tothe present embodiment, therefore, the wheels RR, RL can be controlledseparately. The remaining arrangement is substantially the same as thatdisclosed in FIG. 1, so that its explanation will be omitted. If twomore sets of the three ports-two positions electromagnetic changeovervalve corresponding to the solenoid valve 65 as shown in FIG. 14 areprovided in the embodiment illustrated in FIG. 15, then the tractioncontrol function may be added such as the embodiment in FIG. 14.

It should be apparent to one skilled in the art that the above-describedembodiments are merely illustrative of but a few of the many possiblespecific embodiments of the present invention. Numerous and variousother arrangements can be readily devised by those skilled in the artwithout departing from the spirit and scope of the invention as definedin the following claims.

What is claimed is:
 1. A braking force distribution control system forcontrolling a braking force applied to a pair of rear wheels of anautomotive vehicle in a certain relationship with a braking forceapplied to a pair of front wheels of said automotive vehicle,comprising:a pair of front wheel brake cylinders, each of which isoperatively connected to a respective one of said front wheels forapplying a braking force to the respective front wheel; a pair of rearwheel brake cylinders, each of which is operatively connected to arespective one of said rear wheels for applying a braking force to therespective rear wheel; a reservoir for storing brake fluid; a mastercylinder for pressurizing the brake fluid fed from said reservoir andsupplying hydraulic braking pressure to said pair of front wheel brakecylinders in response to depression of a brake pedal; an auxiliary powersource communicated with said reservoir for pressurizing the brake fluidfed from said reservoir and discharging a power pressure; dynamichydraulic braking pressure regulating means communicated with saidauxiliary power source for regulating said power pressure to a pressureregulated in a certain relationship with the hydraulic braking pressuredischarged from said master cylinder in response to depression of saidbrake pedal; pressure control valve means disposed in respectivehydraulic circuits communicating said regulating means with said rearwheel brake cylinders for controlling the hydraulic braking pressure insaid rear wheel brake cylinders; and control means for actuating saidvalve means to control the hydraulic braking pressure in said rear wheelbrake cylinders in a certain relationship with the hydraulic brakingpressure in said front wheel brake cylinders.
 2. A braking forcedistribution control system according to claim 1, wherein saidregulating means comprises a hydraulic booster for actuating said mastercylinder by said regulated pressure.
 3. A braking force distributioncontrol system according to claim 1, further comprising:wheel speeddetection means for detecting wheel speeds of said front wheels and saidrear wheels; comparison means for comparing the wheel speeds of saidfront wheels and said rear wheels detected by said wheel speed detectionmeans, said control means actuating said pressure control valve means inresponse to a result of the comparison made in said comparison means tocontrol the hydraulic braking pressure in said rear wheel brakecylinders in a certain relationship with the hydraulic braking pressurein said front wheel brake cylinders; termination determining means fordetermining a certain condition for terminating the control of thehydraulic braking pressure by said control means; and pulse increasecontrol means for providing a pulse increase mode to repeat holding andincreasing operations of the hydraulic braking pressure in said rearwheel brake cylinders, wherein said pulse increase control meansprovides a special pulse increase mode to increase the hydraulic brakingpressure in said rear wheel brake cylinders at an increasing rate raisedin accordance with a lapse of time after said termination determiningmeans determines said terminating condition has been fulfilled.
 4. Abraking force distribution control system according to claim 3, whereinsaid pulse increase control means provides a pulse signal having aperiod that varies in accordance with said lapse of time.
 5. A brakingforce distribution control system according to claim 4, wherein saidpulse signal has a constant on-time and such a period as graduallydecreased with said lapse time increased.
 6. A braking forcedistribution control system according to claim 1, furthercomprising:wheel speed detection means for detecting wheel speeds ofsaid front wheels and said rear wheels; standard speed setting means forproviding a certain increasing rate for each of the wheel speeds of saidfront and rear wheels detected by said wheel speed detection means andcalculating a first set speed on the basis of said increasing rate foreach of said front and rear wheels, providing a certain decreasing ratefor each of the wheel speeds of said front and rear wheels detected bysaid wheel speed detection means and calculating a second set speed onthe basis of said decreasing rate for each of said front and rearwheels, and selecting a medium value of said first set speed, saidsecond set speed and said wheel speed of said front wheels or rearwheels detected by said detection means to set a standard speed for eachof said front and rear wheels; and comparison means for comparing saidstandard speed of said front wheels and said standard speed of said rearwheels provided by said setting means, said control means actuating saidvalve means in response to a result of the comparison made in saidcomparison means to control the hydraulic braking pressure in said rearwheel brake cylinders in a certain relationship with the hydraulicbraking pressure in said front wheel brake cylinders.
 7. A braking forcedistribution control system according to claim 6, wherein saidincreasing rate is set on the basis of a certain value of gravitationalacceleration, and wherein said decreasing rate is set on the basis ofeach of the wheel accelerations of said front and rear wheels calculatedfrom the front and rear wheel speeds.